Since the identification of human T cell lymphotropic virus Type III (HTLV-III) (also called lymphadenopathy virus (LAV) or AIDS-associated retrovirus (ARV)) as the probable cause of infectious Acquired Immunodeficiency Syndrome (AIDS) and the establishment of a permissive T cell line for mass production of the virus, substantial progress has been made in characterizing the virus. The complete nucleotide sequence of molecular clones of various provirus isolates have been deciphered. See Ratner et al., (1985) Nature 313, 277-284; WainHobson et al., (1985) Cell 40, 9-17; SanchesPescador et al., (1985) Science 227, 484-492; Muesing et al., (1985) Nature 313, 450-458. The provirus is 9734-9749 base pairs (bp) in length including two long terminal repeat (LTR) sequences. HTLV-III contains many characteristic structural features of other retroviruses: the long terminal repeats, a core protein gene (gag), a gene region (pol) encoding the virion RNA-dependent DNA polymerase and a gene encoding the virus envelope glycoproteins (env). Unlike other retroviruses the HTLV-III viruses contain two additional small open reading frames (SOR-I and 3'ORF) which may play a role in its unusual cytopathogenicity. See Ratner et al., supra.
HTLV-III gag proteins are synthesized in the form of a polyprotein precursor of 512 amino acids which is proteolytically processed to individual gag proteins p17, p24 and p15. Data obtained from the analysis of nucleotide structure and the gag gene products of HTLV-III and ARV indicate that the p17, p24 and p15 proteins are 134, 230 and 122 amino acids long respectively. See Ratner et al., WainHobson et al., Sanchez-Pescador et al., and Muasing et al., supra. Two precursors, p70 and p55, have been detected in HTLV-III infected cells in culture and were shown to share peptide homology with the p24 gag protein Roby et al., (1985). Recently, the p24 protein has been isolated from HTLV-III. Casey J.M., et al., (1985), J. Virol., 55 (2), 417-423.
Several methods for the detection of HTLV-III infection also have been developed. Solid-phase immunoassays employing inactivated HTLV-III as a whole virus antigen immunoadsorbent have been developed for the detection of antibodies against HTLV-III in sera of patients. Such assays have been shown to detect antibodies in more than 80% of sera from patients with AIDS or AIDS-related complex (ARC), or from individuals infected with HTLV-III and these are useful for diagnosing AIDS and for screening contaminated blood.
Assays employing the whole virus, however, have several drawbacks. Large quantities of the virus must be cultivated as supply for test reagents. Although rigorous safety measures can be instituted, there are dangers associated with large scale cultivation of the infectious virus. Further, there exists a risk, however small, that test reagents prepared with the inactivated virus can be contaminated with live virus. Thus, persons who handle the reagents may be subjected to the risk of HTLV-III infection.
Isolated viral p24 protein has been used in an immunoprecipitation assay for detection of anti-HTLV-III antibody in sera of AIDS patients. High serum titer of antibody was found in 73% AIDS patients. Casey, J.M., et al., supra. Assays employing isolated viral components such as the p24 protein, however, do not eliminate the need for production of the virus. Large scale production of the virus is required for isolation of sufficient quantity of the viral protein for preparation of assay reagents.